1. Field of the Invention
This invention relates to enhanced dropwise condensing surfaces and to a method for forming a graphite coating on a metallic substrate which is characterized by a high degree of uniformity of the alignment of the basal planes in the graphite coating with respect to the coated substrate such that the graphite basal planes are essentially parallel to the coated surface.
2. Description of the Prior Art
In the field of condensation heat transfer, there has been a rigorous and continuing search for improved heat transfer surfaces which are characterized by high condensation heat transfer coefficients. Such improvement has been sought because it affords the opportunity to reduce the size of heat exchange apparatus and/or the energy requirements associated with the condensing operation. In particular, much effort has been expended to develop dropwise condensing surfaces, i.e., surfaces which are non-wettable with respect to the vapor component to be condensed, inasmuch as dropwise condensation heat transfer coefficients are in general significantly higher than filmwise condensation coefficients.
In the prior art, dropwise condensing surfaces have variously been produced by permanent bonding of organic coatings to the heat transfer substrate, by absorption of organic dropwise promoting agents on the substrate, and by noble metal, e.g., gold, plating of the substrate. Each of these techniques is characterized by inherent deficiencies which has prevented its broad commercial implementation. Permanently bonded organic coatings generally impose a significant thermal resistance to heat transfer, by virtue of their low thermal conductivity, which obviates any gain attributable to the dropwise character of the coated surface. Absorption of organic promotors on the heat transfer surface provides an enhanced condensing surface for only relatively short periods of service; continued use frequently results in loss of the dropwise character of the surface due to solubilization of the coating in the condensed liquid or to oxidation, hydrolysis, or other chemical reaction involving the organic material which destroys its promotor properties. Gold and other noble metal platings are prohibitively expensive for general commercial utilization.
Accordingly, it is an object of the present invention to provide an improved dropwise condensing surface.
It is another object of the invention to provide such a surface by a substrate coating which is easily formed and permanent in character.
These and other objects of the invention will be apparent from the ensuing disclosure and appended claims.